Trk tyrosine kinase receptors are multi-domain single-transmembrane receptors that play an important role in a wide spectrum of neuronal responses including survival, differentiation, growth and regeneration. They are high affinity receptors for neurotrophins, a family of protein growth factors, which includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5). The neurotrophins share highly conserved structural features, yet their unique amino acid sequences allow each member to elicit high affinity interactions with the extracellular domain of specific Trk receptors, namely TrkA, B or C. Thus, NGF is a preferred ligand for TrkA; BDNF and NT-4/5 are preferred ligands for TrkB; and NT-3 has been shown to bind TrkC, although it also appears to bind TrkA and TrkB with lower affinities.
Among the Trk receptors, the role of TrkB has been well characterized in the central nervous system (CNS). TrkB are widely distributed in the brain, including in the neocortex, hippocampus, striatum, olfactory formation and brainstem. Using BDNF as a cognate ligand, the indispensable roles of TrkB in neuronal survival, differentiation and neuroregeneration have been shown in a number of neurodegenerative models, including stroke, spinal cord injury, axotomy and ALS.
Through various signaling analyses and receptor knockout systems, the responses of BDNF have been shown to depend on the binding and activation of TrkB. As noted, Trk receptors are multi-domain single-transmembrane proteins. They consist of an extracellular ligand binding domain, a transmembrane region, and an intracellular tyrosine kinase domain. The extracellular domain is composed of a leucine-rich motif flanked by two cysteine clusters and two immunoglobulin(Ig)-like domains. Trk receptors have been shown to interact with their ligands mainly through the second Ig-like domain, although contribution of other regions such as a leucine-rich motif and the first Ig domain in ligand-docking has been proposed. The crystal structures of the ligand binding domains of Trk receptors as well as ligand-receptor complexes have been resolved. The ligand-receptor interface appears to consist of two patches: one for a conserved binding motif shared among all neurotrophins and the other specific for each neurotrophin.
Upon the binding of neurotrophins to Trk receptors, receptor dimerization and subsequent conformational changes occur, which are believed to lead to activation of the intracellular tyrosine kinase domain. There are several conserved tyrosines in the intracellular domain of Trk receptors. Phosphorylation of the autoregulatory loop of the kinase domain activates the kinase activity, and phosphorylation of other residues promotes signaling by creating docking sites for adaptor proteins that couple these receptors to intracellular signaling cascades, including Ras/extracellular signal regulated kinase (ERK) protein kinase pathway, the PI3K/Akt kinase pathway and phospholipase C-gamma. Although somewhat overlapping, these individual pathways are involved in discrete biological activities: Ras/MAPK regulates neuronal differentiation and proliferation, PI3K/Akt pathways control actin dynamics and survival and PLC gamma is involved in calcium mobilization.
To date, there exist no successful examples of reagents that act as potent and selective in vivo agonists of TrkB. While BDNF, as a recombinant protein, has been shown to increase neuronal survival and neuroregeneration in a number of CNS degenerative models in vitro and in vivo, the outcomes of BDNF protein therapy in clinics have been negative, most likely because BDNF has a short in vivo half-life. There is therefore a need in the art for pharmaceutical reagents that act as potent and selective in vivo agonists of TrkB.